1. Field of the Invention
The present invention relates to a device for the application of a liquid or viscous coating medium onto a moving surface.
2. Description of the Related Art
Devices for applying coating medium onto a moving surface typically utilize a veil applicator unit/curtain coater which applies the coating medium in the form of a veil or curtain to the moving surface. In the instance of direct application, the moving surface is the surface of a material web, specifically a paper or carton web. In the instance of indirect application, the moving surface is a transfer element which then transfers the coating medium to the material web. A suction device is typically provided for suction removal of air which is carried along by the moving surface and/or the curtain. When coating a moving surface by use of a veil applicator unit, also referred to as the "curtain coating" method, the liquid or viscous coating medium is released by the curtain coater at a predetermined height above the moving surface and drops in the form of a veil or curtain onto the moving surface. Such a coating medium curtain is a relatively unstable formation. Therefore, great care must be exercised to ensure that the conditions under which the coating medium curtain makes contact with the moving surface are such that a high quality coating results. In particular, interruption of the coating medium curtain must be prevented, since this would result in especially strong formation of coating medium splashes. Control of the air boundary layer which is carried along by the moving surface into the area of contact is of central importance. Even if the air boundary layer does not penetrate or interrupt the coating medium curtain, it could prevent uniform coverage of the moving surface with coating medium through formation of air bubbles between the moving surface and the coating medium.
In German patent document 297 11 713, the provision of a blast nozzle disposed prior to the line of impact of the coating medium on the moving surface, when viewed in the direction of travel of the moving surface, is described. The blast nozzle emits an air jet in a direction opposite to the direction of travel of the moving surface. This air jet "cuts" into the air boundary layer, thereby lifting it from the moving surface. Of course, the coating medium curtain is impaired by the turbulence created as a result of the interaction between the air boundary layer and the air jet. The coating medium, therefore, had to be applied in great excess to the moving surface in order to provide a sufficiently stable coating medium curtain.
Under the terms of U.S. Pat. No. 5,624,715, the intrusion of the air boundary layer into the application area is made more difficult by use of a blade which is located a short distance from the moving surface. The blade is followed by a suction device that removes from the application area the remaining air that is being carried along by the moving surface and the air that is being carried along by the coating medium curtain. However, the coating medium curtain may now be impaired not only by an excessively high pressure caused by the boundary air layer, but also by an excessively low pressure caused by the suction device. When utilizing the applicator unit which is described in U.S. Pat. No. 5,624,715, it is necessary to apply the coating medium in great excess onto the moving surface and then scrape off the excess coating medium in order to ensure, respectively, an appropriately stable coating medium curtain and a high-quality coating result. This method requires appropriately dimensioned, large diameter supply and return lines in the area of the curtain coater, as well as in the area of the doctoring device, and appropriately efficient and powerful feed pumps for supplying the excess of coating medium.
A method for the treatment of the coating medium is described in European Patent document 0 517 223 B1, according to which air bubbles are removed from the coating medium in order to achieve a high-quality coating result.